Index Selectivity的意思|示意

For comparison, Figure 3 illustrates the access plan where I forced the DB2 optimizer to ignore the index by adding the clause SELECTIVITY 1 to the predicate in the where clause.

为了便于比较，图3显示了通过将SELECTIVITY 1子句添加到where子句的谓词中，强迫db2优化器忽略索引的访问计划。

Selectivity: When the query optimizer (QO) studies your SQL, the degree of selectivity determines if an index should be used to perform the operation.

选择性：当查询优化器(query optimizer，即QO)读取SQL时，选择性的程度决定了索引是否应该用来执行该操作。

Indexes on the foreign keys referencing tables with few rows have low selectivity, and the utility of requiring the index has always been questionable.

外键上的索引所引用的表行数较少，选择性较少，需要索引的实用程序一直遭受质疑。

The selectivity index of different plant species changes with grazing The selectivity index of different plant species changes with grazing seasons and stocking rate.

山羊对各种植物的选择性指数随放牧季节和放牧率的不同而发生变化。

The selectivity index for height layer varies with seasons and stocking rate.

各高度草层的选择性随季节推移和放牧率不同而发生变化。

There is much less index access and no index AND'ing so that the overall performance can be one or two order of magnitude better - depending on predicate selectivity.

只有相当少的索引访问且没有索引 AND'ing，这样整体性能将提高一个或两个数量级 —— 取决于谓词选择。